Semiconductor devices and ICs are generally contained in semiconductor packages comprising a protective coating or encapsulant to prevent damage during handling and assembly of the components during shipping and when mounting the components on printed circuit boards. For cost reasons, the encapsulant is preferably made of plastic. In a liquid state, the plastic “mold compound” is injected into a mold chamber at an elevated temperature surrounding the device and its interconnections before cooling and curing into a solid plastic. Such packages are commonly referred to as “injection molded”.
Interconnection to the device is performed through a metallic leadframe, generally made of copper, conducting electrical current and heat from the semiconductor device or “die” into the printed circuit board and its surroundings. Connections between the die and the leadframe generally comprise conductive or insulating epoxy to mount the die onto the leadframe's “die pad”, and metallic bond wires, typically made of gold, copper, or aluminum, to connect the die's surface connections to the leadframe. Alternatively, solder balls, gold bumps, or copper pillars may be used to attach the topside connections of the die directly onto the leadframe.
While the metallic leadframe acts as an electrical and thermal conductor in the finished product, during manufacturing the leadframe temporarily holds the device elements together until the plastic hardens. After the plastic cures, the packaged die is separated or “singulated” from other packages also formed on the same leadframe by mechanical sawing. The saw cuts through the metal leadframe and in some instances through the hardened plastic too.
In “leaded” semiconductor packages, i.e. packages where the metallic leads or “pins” protrude beyond the plastic, the leads are then bent using mechanical forming to set them into their final shape. The finished devices are then packed into tape and reels ready for assembly onto customers' printed circuit boards (PCBs).
One example of a leaded package 1 is shown in cross section in FIG. 1A, comprising semiconductor die 4, plastic 2, bond wires 5B and 5C, metallic leads 3B and 3C, and metallic die pad 3A. The metallic leads 3B and 3C and the die pad 3A comprise elements from a single lead frame separated during manufacturing. Leads 3B and 3C along with other leads not visible in the cross section are bent to lie flat or “coplanar” on a PCB depicted by planar surface 6. Owing to the shape of the bent leads 3B and 3C, package 1 is sometimes referred to as a “gull-wing” package.
Such leaded packages are manufactured in a large variety of sizes and pin configurations ranging from 3 leads used for packaging transistors and simple ICs such as bipolar junction transistors, power MOSFETs and shunt voltage regulators, to dozens of leads used for packaging integrated circuits (ICs). To date many billions of products have been manufactured using injection-molded leaded plastic packages. Common packages include small transistor packages like the SC70 and SOT23 packages, small outline packages such as the SOP-8, SOP-16 or SOP-24, and for higher pin counts, the leaded quad flat pack or LQFP. The LQFP, which can have 64 or more leads per package, apportions its leads in even amounts on each of its four edges while SOT and SOP packages have leads positioned on only two sides.
To accommodate the lead bending process, minimum package heights for the SOP and LQFP typically exceed 1.8 mm. Some packages including the small outline transistor package such as the SOT23-3, SOT23-5, SOT23-6 and the SOT223, the small chip package such as the SC70, the TSOP-8 thin small outline package, and the TSSOP-8 thin super small outline package have been engineered for lower profiles, as thin as 1 mm. Below 1 mm thickness it becomes difficult to manufacture any of these packages. Even for greater package heights, maintaining good lead coplanarity during lead bending is a constant concern in the volume manufacturing of gull wing packages.
Accurate forming of leads to tight specifications and tolerances is problematic. Customers consider deformed leads as quality failures, demanding a formal corrective action response and a committed improvement schedule. In extreme cases, manufacturing outside of specified tolerances can result in manufacturing interruptions triggering financial penalties, vendor disqualifications and even litigation.
Poor control of lead bending in manufacturing is not the only limitation of these packages. Despite their ubiquity, leaded injection molded packages suffer from a number of other limitations including poor area efficiency, poor thermal resistance and a relatively thick cross-sectional profile. Specifically, the maximum die size of such packages is small compared to their footprint on a printed circuit board, in part because of the area wasted by the curved portion of the leads. Comparing the maximum die area to the PCB mounting area of the package, area efficiency of gull wing packages can be as low as 30% to 50%.
One way to improve the area efficiency is to bend the leads under the package as shown in the package 11 of FIG. 1B comprising semiconductor die 14, plastic body 12, bond wires 15B and 15C, metallic leads 13B and 13C, and metallic die pad 13A. The metallic leads 13B and 13C and die pad 13A comprise elements from a single lead frame separated during manufacturing. Leads 13B and 13C along with other leads not visible in the cross section are bent under the package to lie flat or “coplanar” on a PCB depicted by imaginary surface 16. The lead shape can be referred to as a “J” lead for its obvious resemblance to the alphabetic character, but in practice is more similar to the gull-wing shape in the inverse direction.
Unlike gull wing package 1, the improved package 11 is able to accommodate a wider plastic body 12 and a larger cavity for die 14 for any given PCB footprint and likewise support a significantly larger die 14, up to three times the die size of gull wing packages with the same footprint, with area efficiencies improving up to 70% or even 80%. Accordingly this package type is referred to as a JW type package, a reference to its J-lead widebody construction. Production volumes to date exceed 1 billion units including the SC70JW, a JW-type package having the same PCB footprint as the SC70, and the TSOPJW, a smaller JW-type package having the same PCB footprint as the TSOP package.
Unfortunately the process of lead bending requires both vertical and horizontal clearance for the lead-forming machine to secure and bend the leads without touching the plastic. Should the machine touch the plastic, the plastic may crack and the resulting product will not pass requisite reliability and hermeticity tests. In order to avoid damage and achieve a lower package profile, the bottom edges of plastic body 12 of JW package 11 are notched to accommodate the upward bend of leads 13B and 13C. Even with this notch, considering minimum tolerances for the bending process, the JW package cannot be reliably manufactured below a thickness of 1 mm. Practically speaking, manufacturing any JW package in volume under 1.1 mm of thickness is challenging.
Aside from their height limitations, gull wing and JW type packages lack effective heat sinking because it is difficult to integrate an exposed die pad as a heat slug into these packages. In low thermal resistance “power packages” a heat slug comprising solid copper is interposed between the die and the PCB, essentially by employing a thick die pad whose backside extends beyond the plastic package. For example, in the package of FIG. 1A, if die pad 3A extended beyond plastic 2 down to planar surface 6, as shown hypothetically by metallic heat slug 7, heat could be more effectively transported from the die into the PCB. In practice heat slug 7 would replace die pad 3A, serving the dual roles of die pad and heat slug. As such, heat slug 7 can also be referred to as an “exposed die pad.”
The problem with heat slug 7 is leads 3B and 3C have to be bent to match the bottom of heat slug 7. Unfortunately, the backside of heat slug 7 is not naturally coplanar with its leads 3B and 3C. While the forming machine can be calibrated to keep leads 3B and 3C relatively coplanar with one another, the distance of the bent foot below the bottom of plastic enclosure 2 will vary and so misalign leads 3B and 3C with the bottom hypothetical heat slug 7. While this misalignment can be tolerated in low volume production, yield fluctuations resulting from natural stochastic variability of a manufacturing process run in high volume are unavoidable, their impact being both risky and potentially very costly.
Moreover, such stochastic variability may result in differing failure modes in an application. For example, statistically, in some cases the bottoms of leads 3B and 3C may extend below the bottom of heat slug 7. In such cases mounting package 1 on a PCB having a top surface 6 will result in electrical connections to leads 3B and 3C, but with the bottom heat slug 7 suspended above plane 6 and unable to conduct heat into the PCB. Conversely, when heat slug 7 extends below the bottoms of leads 3B and 3C, then the leads may not solder onto the PCB whatsoever, resulting in open circuits and defective PCBs. Worse yet, poor or “cold” solder joints may result, passing final manufacturing tests but failing during operation in the field. Significant field failures can result in costly product recalls with the potential for customer damage claims against the manufacturer.
Despite all its manufacturing risks, leaded gull wing package 1 of FIG. 1A is at least conceptually adaptable to accommodate heat slug 7, because the package utilizes a “die up” design, where die 4 sits atop die pad 3A or hypothetical heat slug 7. In contrast, JW-type package 11 of FIG. 1B is completely incompatible with a PCB connected heat slug because the package utilizes a “die down” configuration, one where die 14 sits beneath die pad 13A.
To avoid the variability of lead bending in order to make a low profile package, a completely different type of package, a “leadless” package, was developed and became commercially adopted circa 2000. One common nomenclature for this type of package having leads on all four of its edges is the QFN, an acronym for quad flat no-lead package. A two-side variant of the package, the DFN or dual flat no-lead package is also widely used today. The nominative “leadless” or “no-lead” does not mean the package lacks connections to the PCB, but that its leads do not protrude beyond the edges of the plastic on any side. The term flat implies the package height is uniform, lacking an area devoted to bent leads.
Such a leadless package 21 is illustrated in the cross-sectional view of FIG. 2A comprises semiconductor die 24, plastic body 22, bond wires 25B and 25C, metallic leads 23B and 23C, and metallic die pad 23A. The metallic leads 23B and 23C and die pad 23A comprise elements from a single lead frame, collectively referred to as 23, whereby the leadframe is separated into die pad 23A, leads 23B and 23C, and into other leads (not shown) during manufacturing. As shown the lateral extent of plastic 22 and metallic lead 23C are coplanar with vertical edge 31 such that lead 23C does not extend beyond the edge of plastic body 22. In a similar manner lead 23B is coplanar with the other vertical edge of plastic body 22. Because the leads are vertically oriented, no clearance is required for leads or for lead bending and area efficiencies improve, in some cases up to 80% to 90% PCB area utilization.
The bottom of leadless package 21 is also flat, with the bottom of plastic body 22 coplanar with the bottoms of leads 23B and 23C along horizontal planar edge 26. In the event that optional heat slug 32 replaces die pad 23A, the bottom of optional heat slug 32 is naturally coplanar with the bottoms of leads 23B and 23C shown by horizontal line 26 because leads 23B and 23C and heat slug 32 are all formed out of the same piece of metal.
As shown in the DFN perspective drawing of FIG. 2B, leads 23B, 23D, 23F and 23H do not protrude beyond the cube shape defined by plastic body 22, lying flush with the side face of the package and with the bottom edge defined by horizontal line 26. A plan view of the package underside, shown in FIG. 2C, confirms the coplanarity of leads 23B through 23I with all the edges defined by plastic body 22. The optional heat slug 32 is also naturally coplanar with the bottom of the package and laterally enclosed on all side by a surrounding donut of plastic body 22.
Mounting of a leadless package onto a PCB requires solder between the package and the board, accomplished by coating the PCB with a solder paste prior to component placement. After a pick and place machine places all the components onto the PCB, the PCB is run through a reflow furnace, typically on a movable belt. During the reflow operation, the solder paste melts and adheres only to portions of the PCB where the copper traces are exposed. Likewise the solder only adheres to the exposed metallic surfaces of the mounted components. Accordingly, any PCB assembly manufacturing using solder reflow to attach components is referred to in the industry vernacular as a “reflow” assembly line.
A cross-sectional view of such a PCB assembly is shown in FIG. 2D where DFN package 21 is mounted onto multilayer PCB 27 by solder 30. As illustrated, DFN package 21 comprises, in part, a lead 23B and a plastic body 22. PCB 27 comprises an insulating substrate 28, typically phenolic, and multiple layers of conductive traces 29, 34 and 35, typically comprising copper. Some portions, but not all, of the conductive traces are exposed on the surface. An even smaller portion of conductive traces 29 overlap the conductive leads of the mounted components, in this case lead 23B and optionally heat slug 32.
The solder paste applied to the PCB prior to component mounting typically involves a metal such as Ag or a binary metallic compound such as Pb—Sn that melts at a relatively low temperature. In recent years, Pb compounds have largely been banned for environmental concerns, with higher temperature silver (Ag) solder being used instead. In the reflow assembly process, because the package is placed atop the solder itself, there is no need for solder to be able to flow under the package leads.
During the reflow process solder 30 reflows to where both PCB 27 has an exposed conductive trace 29 and DFN package 21 has an exposed lead 23B. The package itself is held in place purely by surface tension. The solder naturally “flows” away from any area lacking metallic connections on both package 21 and multilayer PCB 27. In essence, the solder goes to where it is needed, i.e. to the solder joint, and after reflow solder is absent from all other portions of the PCB. In cases where optional heat slug 32 is present, solder 33 will also remain in this region.
Ideally, after reflow, solder 30 fills the entire cavity between the bottom of lead 23B and conductive trace 29. In some cases solder 30 will “wick” up onto the side of lead 23B through the fluidic behavior to reduce surface tension. The connection to the side of lead 23B is inconsistent and cannot be relied on to insure a good connection. Instead, the solder located in the region directly between the bottom of lead 23B and conductor 29 must facilitate the main electrical and mechanical connection of the mounted component. In the case where optional heat slug 32 is present, solder will ideally fill the entire cavity between the bottom of heat slug 32 and the top of PCB conductive trace 34.
If any of these intervening regions are not filled with solder 30, an undesirable void will result, weakening the mechanical strength of the solder joint, increasing the electrical resistance of the connection, and possibly compromising the reliability of the product. For example, solder joints with large voids mechanical weaken in bond strength through repeated contractions and expansions during thermal cycling or power cycling. Eventually, a crack will form in the solder, and the component will develop and open or intermittent electrical connection. In extreme cases the component may “fall off” the PCB altogether.
So it is critical to confirm that a void free solder joint is formed on every lead during manufacturing. Since, however, the main solder connection is “beneath” the component, visual inspection is not possible, so that expensive x-ray inspection equipment is required.
PCB assembly using the described reflow method is commonly available in modern multilayer PCB factories used for manufacturing smartphones, tablets, notebooks, servers, and network infrastructure. Such products typically command sufficiently high prices to afford the higher manufacturing cost of a multilayer PCB assembly facility. Some markets, however, are extremely cost sensitive and cannot afford the high cost of multilayer PCBs or reflow PCB assembly with x-ray inspection. Instead, the cost sensitive products are still manufactured in PCB facilities built in the 1960s and using fully depreciated equipment.
Examples of products demanding low cost PCB assembly include most consumer products including clocks, radios, televisions, and home appliances along with many power supply modules used in consumer, lighting, HVAC (heating, ventilation and air conditioning) and industrial applications. These old PCB factories are incapable of reflow solder manufacturing, lack x-ray inspection equipment, and can accommodate only one- or at most two-sided PCBs, i.e. PCBs with only one or two conductive layers. In fact the cost of a multilayer PCB manufactured with reflow processing can be 2 to 5 times that of single-layer PCBs made in these older factories.
To attach the components to the PCB, these low-cost factories use a method known as “wave soldering”, essentially immersing the board and its components with molten solder that sticks only where a solder joint is to be made, i.e. where the component and the PCB both have exposed metallic surfaces in close proximity. In practice the components are “glued” down in their proper place and then the PCB is dipped in a molten solder bath. Because the solder is applied after the component is mounted, no solder is present between the bottom of a package lead and the PCB conductive trace as it is in FIG. 2D. Instead the solder attaches itself to the sides of the leads and wicks its way up onto the lead. In order to make a good connection the solder must cover a significant portion of any component's exposed leads. Because the solder is present on the exposed leads, the soldering operation can be inspected visually without the need for expensive x-ray inspection equipment. For this reason, only leaded packages such as the gull wing and JW-type packages shown in FIG. 1A and FIG. 1B are used with wave-solder PCB assembly.
Despite their performance advantages, leadless packages are intrinsically incompatible with wave soldering and low cost PCB assembly. As described previously, in wave soldering there is no means by which solder can squeeze between the bottom of the package leads and the PCB conductive traces, since those areas are filled with glue. Likewise, the vertical edges of the exposed leads 23B and 23C of the leadless package 21 shown in FIG. 2A are not suitable for wave-solder assembly because the solder will not reliably wick its way onto a vertical exposed lead, in essence because it is too steeply inclined. Also, as a vertical edge, it is impractical to visually inspect solder coverage of the lead.
Unfortunately, the angle of the side of a leadless package is necessarily vertical because of the way in which it is manufactured. FIG. 3 illustrates a leadless package during manufacturing after plastic molding but before singulation. As before, leadless package 21 comprises die pad 23A, die 24, bond wires 25B and 25C and leads 23B and 23C encapsulated in a plastic body 22. To the right of package 21 is an identical package manufactured with the same leadframe. As shown, lead 23C actually is a solid piece of metal extending into the next package.
The region labeled “saw blade” describes where the saw cuts during die singulation. During sawing, it cuts through plastic 22 and ultimately through lead 23C separating package 21 from its neighbors. Because the edge of package 21 is defined by a sawing operation, the edge of the saw blade 31 is necessarily substantially vertical. The coplanarity of lead 23C and the remaining plastic body 22 is a natural result of the fact that edge 31 is defined by the saw blade's cut. No practical way exists to slope the cut line of the saw blade, so edge 31 is necessarily vertical. As a result, all leadless packages today lack the ability to be assembled using low cost wave solder PCB factories.
In conclusion, leaded packages such as the gull wing, JW-type, and LQFP plastic packages are compatible with low cost wave-solder PCB assembly and visual inspection, but suffer from a high package height, problems with maintaining precise coplanarity of the leads during the bending process, poor area efficiency, and an inability to incorporate a heat slug for improved power dissipation. Leadless packages like the DFN and QFN offer superior coplanarity, high area efficiency, and the ability to incorporate a heat slug for improved power dissipating capability, but are incompatible with low cost PCB assembly using wave-soldering and visual inspection, instead requiring more expensive solder reflow PCB manufacturing with x-ray inspection.
What is needed is a new package that offers the performance and coplanarity benefits of the leadless package but is compatible with the low-cost PCB assembly method that uses wave soldering and visual inspection.